Notebook asset drag

Fixed issue #623.
Ability to drag single asset from local file system to Asset panel in Rcloud.
You are creating a global variable here (`upload_asset` gets attached to `window`).  We try to avoid that as possible, and especially not by just leaving off any scope.  It looks like here it will work just to put a `var` before the variable name (and a nit, put a space before the `=`), because this function is only used within this scope... or equivalently, make it `function upload_asset(to_notebook)`